(1) Field of the Invention
The present invention relates to a micro-dimensional apparatus for optically measuring a dimension between opposed edges formed on a substrate, with a zone between the opposed edges and a surface of the substrate having a uniform reflectivity, the reflectivity of the zone between the opposed edges being different from that of the surface of the substrate, and more particularly, to such a micro-dimensional measurement apparatus wherein the measurement is carried out by scanning substrate with a light beam having a predetermined light intensity distribution, such as a Gaussian distribution
(2) Description of the Related Art
Developments in precision manufacturing now demand a very high accuracy in the working of various precision components. For example, in the manufacturing of a precision component such as an integrated circuit, a magnetic head or the like, the working accuracy must be on the order of less than 1 .mu.m. Accordingly, there is a pressing need for a micro-dimensional measurement apparatus by which a fine precision component can be dimensionally measured with a high accuracy and reliability.
In this field, it is well known to measure the dimension of a fine object by an optical measurement system with includes a white light source for illuminating the fine object, a microscope for magnifying an image of the illuminated fine object, a television camera for reading optical information as video data from the magnified image, and a processor for processing the video data to calculate a dimension of the fine object.
For example, where the width of a fine gap formed between magnetic portions of a magnetic head is measured by an optical measurement system, the area of the magnetic head which includes the fine gap to be measured is magnified by the microscope under the illumination of the white light source, and optical information is then read by the television camera as a series of video data from the magnified image. The series of video data is processed by the processor so that a light intensity pattern of the light reflected from the measured area is prepared with respect to a series of addresses of picture elements read by an image sensor of the television camera. In other words, the reflected light intensity pattern so obtained can be considered to be a function of a distance measured along a line which crosses the fine gap at the measured area. Since the gap zone of the magnetic head has a lower reflectivity than that of the magnetic portions thereof, the reflected light intensity pattern has a minimum peak which corresponds to a middle point between the gap edges. In particular, the reflected light intensity pattern shows a profile of a curve descending gradually toward the minimum peak and then ascending gradually therefrom.
In this prior optical measurement system, in order to measure a width of the gap zone, a slice pitch obtained by slicing the reflected light intensity pattern at a predetermined slice level is calculated by the processor. This slice pitch corresponds to the number of picture elements read by the television camera at the gap zone along the line crossing the measured area. Accordingly, if some reference slice pitch data obtained from known widths of sample gap dimensions in the same manner as mentioned above are previously prepared, it is possible to calculate a width of the gap zone from the measured slice pitch on the basis of the reference slice pitch data.
Nevertheless, the optical measurement system as mentioned above suffers from drawbacks brought by the use of the white light source. In particular, it is impossible to carry out the measurement with a high accuracy and reliability because it is difficult to stabilize an intensity distribution of the white light source with the passage of time. Also, when a fine dimension on the order of less than 1 .mu.m is measured, the accuracy of the measurement is not satisfactory because it is very difficult to obtain a fine spot for the illumination from the white light source, so that the light picked up from the fine gap zone is affected by the light reflected from the zone other the measured area.
British Patent No. 2147097 discloses another type of optical measurement system for dimensionally measuring a fine object, which system includes a laser light source for emitting a laser beam having a Gaussian distribution, an acoustic-optical device for a stepped deflection of the laser beam, to scan the fine object to be measured with the laser beam, a detector for detecting the laser beam reflected from the fine object, and a processor for processing the reflection data obtained from the detector to calculate a dimension of the fine object.
This prior optical measurement system is directed to measuring a dimension between opposed edges formed on a substrate by scanning the substrate with a laser beam under the condition that, when the laser beam is projected on an edge line of the opposed edges, a portion of the projected laser beam at one side of the edge line is detected by the detector, but the other portion thereof at the other side of the edge line is not detected by the detector. For example, if an element having a trapezoid cross-section (i.e. an IC conductor element) is provided on the substrate, it is possible to measure a dimension between opposed edges of such an element by this prior optical measurement system, because these edges satisfy the condition mentioned above, in that when the laser beam is projected, the part of the laser beam impinging on an oblique face extending from each of the edge lines toward the substrate surface is not detected by the detector.
In this measurement, a substrate having the element as mentioned above is step-scanned while the laser beam is controlled by the acoustic-optical device, so that the laser beam crosses each of the edge lines of the element. The laser beams reflected from the substrate at the scanning steps are detected by the detector as a series of reflection data, and the processor prepares a reflected light intensity pattern on the basis of the series of reflection data with respect to a series of deflection voltage values, each of which is applied to the acoustic-optical device to deflect the laser beam at each of the scanning steps. Also, the processor arithmetically processes the reflected light intensity pattern to determine two positions of the edges of the element. In other words, two deflection voltage values corresponding to the two positions of the element edges are determined by the processor. Accordingly, it is possible to calculate a dimension between the element edges from a difference of voltage between the two deflection voltages, which corresponds to a distance of deflection between the two positions of the element edges.
The optical measurement system as disclosed in the above-mentioned British patent is directed to the measurement of a dimension of a fine object which is larger than a spot diameter of the laser beam, which can generally be reduced to the order of 1 .mu.m, and accordingly, the measurement of such a fine object can be satisfactorily carried out with a high accuracy and reliability. But when a dimension to be measured between edges of the fine object is less than 1 .mu.m, the accuracy and reliability of the measurement is considerably reduced.
The pending U.S. Pat. No. 014619, filed by the same applicant, discloses two types of micro-dimensional measurement apparatus similar to the British patent mentioned above. One of these micro-dimensional measurement apparatuses is directed to an improvement of the optical measurement system disclosed in the British patent, so that a dimension between opposed edges formed on a substrate can be measured with a high accuracy and reliability, although the dimension to be measured is less than a spot diameter of the laser beam. The other of the micro-dimensional measurement apparatuses is also arranged to be able to measure a dimension less than a spot diameter of the laser beam with a high accuracy and reliability, but the measurement is carried out on the condition that a zone between the opposed edges and a surface of the substrate have respective uniform reflectivities which are different from each other.
In the micro-dimensional measurement apparatus mentioned later, the substrate is scanned with the laser beam controlled by the acoustic-optical device in the same manner as mentioned above, and the laser beams reflected from the substrate at the scanning steps are detected by the detector as a series of reflection data. The processor prepares a reflected light intensity data pattern on the basis of the series of the reflection data, and then reads at least two data from the reflected light intensity data pattern. The processor stores two kinds of reference data corresponding to the two read data, and calculates a distance value as a true dimension to be measured between the opposed edges from the two kinds of sample data on the basis of the two read data. The two kinds of reference data are read from a series of reflected light intensity sample patterns previously prepared on a theoretical basis and/or an experimental basis with respect to sample distance values selected as a dimension to be measured between the opposed edges.